Combustion processes which employ oxygen as oxidizing agent were always used in the past when increase in the efficiency of a furnace unit, savings of primary energy or reducing the volume of the combustion exhaust gases were required. In addition to these main advantages of use of oxygen, the ignition behavior and flame stability were improved and higher process temperatures can be achieved.
The combustion processes were optimized in the past principally with respect to the degree of combustion, in that the resulting CO gases and CH compounds were completely burned.
The adiabatic flame temperatures which are higher by 800.degree. to 1000.degree. C. when technical-grade oxygen is used as compared to a fuel/air combustion lead to more intensive nitrogen oxide (NOx) formation.
The nitrogen oxides arising as by-products in the combustion are considered to be a cause of acid rain and increased ozone formation (harming of forests and people).
Although the dependencies and qualitative shares of the NOx formation mechanisms have not yet been completely clarified, it is currently considered to be confirmed that the flame temperature &gt;1300.degree. C. the residence time of the oxygen at high temperatures and the oxygen partial pressure are the chief influencing factors on the thermal NOx formation.
At the same time, NOx is formed on the basis of the nitrogen organically bound in the fuel. In addition, promt-NO- formation occurs, by free oxygen reacting during the course of the combustion.
The principal requirements which are made of low-pollutant combustion, such as stable ignition behavior, making possible large control ranges without flame instabilities and ensuring complete combustion, have decisively influenced the combustion concepts and aspects of the necessary burner structures.
The previously known low-pollutant combustion methods and the associated combustion devices are NOx-reducing measures for near-stoichiometric combustion, avoidance of flame temperature peaks by recirculation of combustion exhaust gases to the combustion zone and multi-stage combustion by dividing the amount of oxygen required into at least two part-streams.
Scientific studies have been able to confirm in the last decade that the flame temperature peaks are the main cause of thermal NOx formation. From this resulted the requirement of creating effective flame cooling. The flames can be cooled by recirculation of combustion exhaust gases and adding them to the combustion zone or by means of internals in the combustion zone. The internals are exposed to intense heat effects so that they are subjected to high wear.
In a known combustion concept for low-pollutant gas combustion with oxygen, a high-impulse flow of the main oxygen stream was provided for recirculation of the combustion exhaust gases.
To generate the high-impulse flow of the main oxygen stream, Laval nozzles have been proposed (U.S. Pat. No. 5,104,310) in which the pressure energy is converted into velocity.
Disadvantages which result are the high manufacturing costs of these burners and the unsatisfactory control range of the burner. Low-pollutant gas emissions are principally achieved for the design conditions, since the Laval nozzles only operate optimally in a narrow range.
In the known combustion methods for low-pollutant gas combustion, at least 5 to 10% of the oxygen required for stoichiometric combustion is required for flame stabilization. As a result of these large amounts of oxygen, NOx is additionally formed in the region of the flame root.
The use of burner bricks for flame stability with simultaneous reduction of the primary oxygen amounts to 1 to 5% leads to additional costs, since the burner bricks are composed of high-grade materials and are exposed to accelerated wear because of the high thermal stresses.
The methods previously known for low-pollutant gas combustion only partially fulfill the requirements made with respect to decreased NOx pollutant gas emissions with simultaneous economic utilization of the fuel (no unburnt fuel). They thus operate economically only in a restricted range.